The present invention relates to a method for fabricating a semiconductor device, and more particularly relates to a method for fabricating a semiconductor device including the process step of dry-etching a silicon dioxide film.
As a semiconductor device has been downsized, the size of a contact has been reduced, but the thickness of an interlevel dielectric film has not. Accordingly, the ratio of the thickness of an interlevel dielectric film to the size of a contact (i.e., an aspect ratio) has considerably increased recently. In view of these circumstances, it is important to establish some technique of forming a contact hole with a high aspect ratio by etching.
In order to form a contact hole with a high aspect ratio by etching, dry etching technique using plasma including fluorocarbon gas, in which the ratio of fluorine to carbon per molecule is relatively small, has been vigorously researched and developed.
However, in the case of forming a contact hole by using plasma including fluorocarbon gas of such a type, if the selectivity with respect to a photoresist is set higher, then a polymer film mainly composed of carbon and fluorine is deposited on the bottom of such a contact hole. If a contact is formed by filling in the contact hole with a conductive member without removing such a polymer film, then the resulting contact cannot exhibit satisfactory characteristics. In order to cope with such a problem, various methods for removing such a polymer film by using oxygen plasma have been proposed. However, the present inventors found that if oxygen plasma processing is performed to remove a polymer film from the bottom of a contact hole, then resulting contact resistance increases to the contrary. Also, when a contact hole is formed in a silicon dioxide film by using a fluorocarbon gas such as C.sub.4 F.sub.8, the depth of a contact hole that can be formed on the same etching conditions (hereinafter, such a depth will be referred to as an "etching depth") adversely decreases as the number of wafers etched increases.